Date:         Sat, 6 Dec 1997 12:43:14 -0400
Reply-To:     mark keller <kelphoto@BRIGHT.NET>
Sender:       Vanagon mailing list <Vanagon@Gerry.SDSC.EDU>
From:         mark keller <kelphoto@BRIGHT.NET>
Subject:      Engine Vacuum Gauge Diagnotics Uses.
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USES

I wanted to see how hard my little VW engine was a working at 70 mph, and
to be aware of not over working it when the wind was up against me. Plus I
just like having a vacuum gage to look at. It's moves as much as a
tachometer does. In fact they are so sensitive that when you turn on the
lights you'll see the load increase of the alternator on the engine
immediatly.


A vacuum guage is a superlative diagnostic device, the equivelent of a
multimeter in many ways. See below. Vacuum gauges are use in most light
piston engine aircraft to "tune the engine fuel mixture" for cruise. The
benefit for us is that you can get a real good idea the internal condition
of you engine and how hard you working your engine when driving under
various conditions. Rather than go into a big "hows come", if you want to
"see" how your engine is performing it's a great tool. I've include
diagnostic procedures from my vacuum test guage manual, which is what
vacuum gauges are also "supposed" to be used for on car engine diagnostics.

 I use this guage on all motors that come to me for diagnostic of  internal
problems first.  Good mechancis can just listen or feel their way around a
motor they are familiar with. The guage takes a lot of the guess work out
for me. For you and me, it give advance warning of problems, before idiot
light start flashing.

BEGIN with a warm engine at slightly higher than normal idling speed 1200
RPM should be fine.

NORMAL ENGINE- the needle shoud be STEADY between 17 and 21, this will be
lower for a 4 cyc than an 8 or 6 cyl.  Mine is about 15.

LEAKING PISTON RINGS-  The guage behavior will be very much like a normal
engine ie steady, but 3 to 4 inches lower, I'd say 11 or so.

SLOW TIMING- If the compression is known to be good and yet the needle
reads lower that it should, the ignition timing may be slow. Verify with a
timing light. A considerably lower than normal reading may be due to slow
valve timing. That last part is probaly for the timing chain engines which
have strecth problems. That's why they run so good after you replace the
timing chain.

LEAKING INTAKE-  A steady needle but extremely, low reading indicates a
proabable air leak in the carburetor, I'll add injector O- rings too,
intake manifold or gaskets.

LEAKING HEAD GASKET- If the needle fluctuates regularly between a high and
a low reading, the cylinder head gasket has probably blown out btween two
adjacent cylingers, verify with a compression test.

SPARK PLUG GAPS- A needle fluctuation over a much narrower range perhaps
about 2 inches incicates that the spark plubg gaps may be spaced too close.

DEFECTIVE VALVE ACTION- If the needle vibrates rapidly at the idle speed,
the intake valves guides are probably worn.  If the needle vibrates rapidly
when the engine is accelerated, there is probably one or more weak valve
springs.  An intermittent dorp of 3 or 4 points indicates sticking valves
wheras a regular drop indicates a burned or leaking valve.


 Short cuts like spraying carb cleaner around intake seals can give a rise
in engine RPM and thus indicate a where the leak is. Also  you can use a
pair of needle nose pliers to pinch a suspected hose. If the guage jumps up
then there was a leak. No change, no leak.


Sincerely Mark
91 Carat WE
"Lazarus"